1. Field of the Invention
The present invention relates to a method for producing a single crystal by pulling the single crystal from a molten material in a crucible using the Czochralski (CZ) method, and more specifically to a method for producing a single crystal by applying an axially symmetric, radial cusp magnetic field to the molten material in the crucible using a magnetic field applying means installed outside the furnace and having magnets of the same polarity facing each other on upper and lower portions thereof.
2. Description of the Background
In the process of pulling a single crystal using the CZ method, for producing a single crystal of silicon and the like, the single crystal 3 is pulled while rotating from a molten material 2 contained in a quartz crucible 1 as shown in FIG. 1. In this case, oxygen dissolves in the molten material 2 contained in the crucible 1, from the quartz of the crucible 1, resulting in the incorporation of oxygen into the single crystal 3. Although the oxygen incorporated in the single crystal 3 cannot be said to be a harmful element because it enhances the mechanical strength of the crystal, it may cause various crystal defects and may lower the device properties when semiconductor devices are fabricated, if the quantity of oxygen is too large. In any case, control of the oxygen concentration to a target value in the pulling direction or in the radial direction of the single crystal 3 is a critical technical problem in the pulling of the single crystal 3 by the CZ method.
As one of the techniques for solving this problem, a method to apply a cusp magnetic field to a molten material in a crucible has been proposed in Japanese Patent Publication No. 2-1290. In this method, a magnetic field applying means having magnets of the same polarity 4, 4 facing each other on upper and lower portions thereof is installed outside the furnace as shown in FIG. 1, thereby applying an axially symmetric, radial cusp magnetic field to the molten material 2 in the crucible 1. This method features: the repulsion of upper and lower magnetic fields in the vicinity of the liquid surface of the molten material 2 to apply an axially symmetric magnetic field bent at a substantially right angle; the inhibition of convection of the molten material 2 in the crucible 1 by the component of the magnetic field intersecting the liquid surface of the molten material 2 at a right angle, and the component of the magnetic field intersecting the side wall and bottom of the crucible 1 at a right angle; and control of the relative position of the magnetic field distribution for the crucible 1 by the position of the magnets.
In the crucible 1, the molten material 2 produces convection along the internal surface of the crucible 1 as shown by arrows 5, 5. Although fresh molten material 2 is supplied to the vicinity of the internal surface of the crucible 1 due to this convection, whereby the dissolution of oxygen from the internal surface is accelerated, a magnetic field intersecting the side wall and the bottom of the crucible 1 inhibits convection along the internal surface of the crucible 1, controlling the dissolution of oxygen from the internal surface of the crucible as a result of the controlled convection by varying the intensity of the magnetic field. Also, a magnetic field component intersecting the liquid surface of the molten material 2 at a right angle, inhibits the flow of the molten material 2 in the vicinity of the liquid surface, controlling the evaporation of oxygen from the liquid surface. Since the quantity of oxygen in the single crystal 3 is determined by the balance of the transportation of oxygen by convection, the quantity of quartz dissolved, and the quantity of oxygen evaporated, the quantity of oxygen in the single crystal 3 is properly controlled by controlling these factors.
Various methods for improving the pulling of single crystals utilizing this cusp magnetic field have also been proposed. For example, Japanese Patent Application Laid-Open No. 1-282185 discloses a method for making uniform the oxygen concentration distribution in the radial direction of a single crystal by rotating the crucible in the direction opposite to the rotation of the crystal, at a specific speed determined by the sizes of the crucible and the crystal, the quantity of the molten material in the crucible, and the intensity and the distribution of the applied magnetic field.
Japanese Patent Application Laid-Open No. 5-194077 (Japanese Patent Publication No. 8-18898) discloses an improvement of the above method for increasing the rotation speed of the crucible and decreasing the intensity of the cusp magnetic field with the progress of single crystal pulling, in the process of pulling a single crystal from a molten material to which a cusp magnetic field is applied. According to this method, the uniformity of the oxygen concentration in both single crystal pulling and radial directions can be controlled more accurately than the method for controlling the rotation of the crucible without applying a cusp magnetic field to the molten material disclosed in Japanese Patent Application Laid-Open No. 57-135796 (Japanese Patent Publication No. 3-21515).
The previously proposed methods for pulling single crystals using a cusp magnetic fields feature the control of the intensity of magnetic fields as well as rotation speed of crucibles in order to make oxygen concentration in the single crystals uniform. In practice, the intensity of magnetic fields and rotation speed of crucibles are operational parameters for controlling oxygen concentration, and for example, by varying the rotation speed of the crucible with the progress of single crystal pulling, the oxygen concentration in the direction of single crystal pulling can be made uniform.
However, the inventors of the present invention found that control of the rotation speed of the crucible under an applied magnetic field, especially varying the rotation speed of the crucible with the progress of single crystal pulling, is more sensitive to the oxygen concentration in the crystal than in the case of conditions without a magnetic field, making it difficult to control the oxygen concentration accurately. That is, although the oxygen concentration in the crystal is lowered by applying the magnetic field, varying the rotation speed of the crucible is not as effective as expected for making uniform the oxygen concentration distribution in the direction of single crystal pulling. Furthermore, it was found that varying the rotation speed of the crucible under an applied magnetic field tends to produce a significantly adverse effect, damage of the bottom of the crucible, due to melting or the polycrystallization of the single crystal, compared with the case of conditions without a magnetic field.
In the method disclosed in Japanese Patent Application Laid-Open No. 5-194077 (Japanese Patent Publication No. 8-18898), the rotation speed of the crucible is increased while gradually decreasing the intensity of the magnetic field with the progress of single crystal pulling. By this method, although oxygen concentration in the direction of crystal growth (the direction of single crystal pulling) becomes uniform, the following problems also arise.
The irregularity of oxygen concentration in the direction of single crystal pulling is believed to be caused mainly by a decrease in the dissolution of oxygen into the molten material, decreasing the oxygen concentration in the molten material, due to a decrease in the quantity of the molten material in the crucible with the progress of single crystal pulling which decreases the contact area between the molten material and the crucible. Here, if the rotation speed of the crucible is increased with the progress of single crystal pulling, the tendency of the molten material containing a large amount of oxygen present in the vicinity of the bottom of the crucible, reaching immediately underneath the crystal, increases, gradually increasing the oxygen concentration in the single crystal. As a result, the decrease in oxygen concentration with the progress of single crystal pulling is offset.
This method is effective both without an applied magnetic field, and with an applied magnetic field. Under an applied magnetic field, since the convection of the molten material is inhibited, the accuracy of control of the oxygen concentration is considered to be higher than in the case of no applied magnetic field. However, under actual conditions, the control of the oxygen concentration is difficult, and high controlling accuracy is difficult to achieve.
Also, although the convection mode of the molten material is directly changed by varying the rotation speed of the crucible during pulling of the single crystal, this tends to accelerate the damage caused by melting of the bottom of the crucible under an applied magnetic field. This means not only that the usable life of the crucible is shortened, but also increases the possibility of polycrystallization, because quartz in the crucible is mixed into the molten material in the form of particles or debris without completely dissolving, this debris floating due to the rising flow, and reaching the interface of the crystal.
In this method, the intensity of the magnetic field is 0 at the rear half of the straight portion of the single crystal (after solidification percentage, g=50-80%). The reason is that a decrease in the oxygen concentration by the pulling is inhibited. However, making the magnetic field 0 at the rear half of the straight portion of the single crystal means that the conventional pulling without applying magnetic fields is performed in this area, and causes the promotion of a phenomenon in which the internal surface of the quartz crucible is shaved by the convection of heat, and raised to the growing interface. As a result, polycrystallization, a big problem in the conventional CZ method, is easily caused. This has also been proven by the experiments by the inventors of the present invention, demonstrating that the rate of formation of single crystals is lowered (by about 65%).
As described above, although the method for varying the rotation speed of the crucible during single crystal pulling makes uniform the oxygen concentration distribution in the direction of single crystal pulling, even under an applied magnetic field, its accurate control and the extension of the usable life of the quartz crucibles accompanying an increase in the diameter and weight of single crystals, are more difficult than in the case of conditions without an applied magnetic field, although the convection of the molten material is inhibited.